Imported PNG shows black instead of transparency in the background
The image you uploaded contains no alpha channel.



You can check the alpha channel for the image using the channel display switches at the bottom of the UV/Image editor window



Blender will not magically create an alpha channel for you, if the image contains none.



A half-ass solution (without getting into editing in an image editing app, or sourcing an image with a proper alpha channel), is to use the black and white representation of the image itself as a mask to control the mix between an emission and a transparent shader, so that the white pixels of the image will be emissive and the black ones transparent.